In recent years, along with progress of high-integration and downsizing of semiconductor elements, heat generation amount at a time of using such semiconductor element simply increases, and efficient dissipation of such heat has become a problem to be solved. For this purpose, as a circuit substrate for power module, a circuit substrate is used, which comprises a ceramic substrate such as an aluminum nitride substrate or a silicon nitride substrate having high insulation property and high thermal conductivity, a metal circuit made of copper or aluminum formed on the ceramic substrate, and a metal heat dissipation plate made of copper or aluminum formed on the rear surface of the substrate.
A typical heat dissipation structure of conventional circuit substrate is that a base plate is soldered to a rear surface (heat dissipation surface) of a circuit substrate via a metal plate such as a copper plate, and such a base plate is made of copper. However, in this structure, there has been a problem that when a heat load is applied to a semiconductor device, a crack of solder layer due to the difference of thermal expansion coefficient between the base plate and the circuit substrate occurs, and as a result, heat dissipation becomes insufficient to cause malfunction or destruction of a semiconductor element.
To cope with such a problem, an aluminum alloy-silicon carbide composite is proposed, which has a thermal expansion coefficient close to that of a circuit substrate (Patent Document 1). However, Patent Document 1 does not describe as to the shape, particularly, a warped shape of the aluminum alloy-silicon carbide composite, and accordingly, when it is actually used as a base plate for power module, there is a case that sufficient heat dissipation property is not obtained.
A base plate is used as it is joined with a heat-dissipation fin in most cases, and the shape and warpage of the joined portion are also important properties. For example, when a base plate is joined with a heat-dissipation fin, usually a heat-dissipation grease having high thermal conductivity is applied to portions to be joined, and the base plate is fixed to e.g. a heat-dissipation fin or a heat-dissipation unit by screws through holes provided in the peripheral portion of the base plate. However, if many fine irregularities are present on the base plate, a gap is formed between the base plate and the heat-dissipation fin, and even if a heat-dissipation grease having high thermal conductivity is applied, thermal conductivity is significantly decreased. As a result, there has been a problem that heat dissipation property of entire module constituted by the ceramic circuit substrate, the base plate and the heat-dissipation fin, etc. is significantly decreased.
To cope with this problem, in order to reduce such a gap between the base plate and the heat-dissipation fin as much as possible, a base plate formed to have a convex warpage in advance is used. This warpage is obtainable is by applying a pressure to the base plate as it is heated, by using a jig having a predetermined shape. However, since warpage obtained by such a method varies in the warpage amount and shape, and thus there has been a problem that its quality is not consistent. Further, due to the variation of warpage shape or surface irregularities, there has been a problem that a large gap is still formed between the base plate and the heat-dissipation fin.
There is a method of forming a warpage by cutting the base plate surface by machining. However, in this method, since the aluminum-silicon carbide composite is extremely hard, significant amount of grinding by a tool such as a diamond is required, and there has been a problem that its cost is high. To solve the above problem, a method is proposed, in which a flat silicon carbide porous body is impregnated with a metal containing aluminum as the main component, an aluminum alloy layer made of a metal containing aluminum as the main component is formed on each primary plane, and the aluminum alloy layer in the heat dissipation side is mechanically fabricated.
However, in a case of producing a base plate by using this method, when the aluminum alloy layer is mechanically fabricated, it is necessary to fabricate the aluminum alloy layer so that the aluminum-silicon carbide composite is not exposed to the outside. For this reason, considering warpage or distortion of the base plate itself and variation of its fabrication, the thickness of aluminum alloy layer to be formed on each primary plane needs to be inevitably thick, and for this reason, the thermal expansion coefficient of the base plate itself increases, and when it is soldered with a ceramic circuit substrate at a time of assembling a power module, there has been a case where a recess is formed on a heat dissipation plane of the ceramic circuit substrate.
Further, in this method, since it is necessary to control the thickness of the aluminum alloy layer on each primary plane to have uniform thickness and not to make the aluminum-silicon carbide composite exposed to the outside, there has been a problem that high degree of fabrication technique is required.
Patent Document 1: JP-A-5-507030